Jitendra Sangwai

Semiclathrate hydrates of natural gas have shown potential applications in natural gas storage and transportation. Promoters, viz., tetra-n-alkyl ammonium bromide (TBAB) and tetrahydrofuran (THF) have positive impacts on the phase stability condition in lowering the required pressure for hydrate formation. As part of this work, a predictive model for the phase stability of gas hydrate, which are necessary to understand the phase behavior of methane (CH4) hydrate in promoters, has been proposed. The fugacity of hydrate former in the gaseous phase is calculated from Peng-Peng-Robinson equation of state (PR-EoS), while the fugacity of water in the liquid phase is computed from recently proposed Pitzer-Mayorga-Zavitsas-Hydration (PMZH) model for TBAB system and non-random two liquid (NRTL) model for THF system. The van der Waals Plattew model is employed for the hydrate phase. The vapor pressure of water in the empty hydrate lattice as well as Langmuir adsorption constants have been expressed in terms of concentration of the promoters. The predictions of the proposed model are found to be match well with experimental data on phase stability of CH4 hydrate formed using TBAB and THF aqueous systems. Furthermore, the developed model is employed for the prediction of phase stability conditions of the semiclathrate hydrates of CH4 in TBAB + NaCl system. The developed model is found to interpret the promotion effects of both TBAB (with or without NaCl) and THF on phase stability conditions of CH4 hydrate. AARD-P% with PMZH model are observed to be 3.21% and 8.73% for semiclathrate hydrates of CH4 in TBAB and TBAB + NaCl, respectively, and 8.56% for clathrate hydrate of CH4 in THF. The model may be extended to evaluate the phase stability conditions of hydrates of multicomponent gas systems in TBAB/THF which are necessary for real field applications.

With the growing need to explore non-conventional energy sources, the hydrates of natural gases offer a realistic solution in the need for alternative energy sources. Gas hydrates are typically entrapped in the porous media showing sensitive phase stability conditions. Models for phase stability for gas hydrate have not yet been extensively investigated for porous media and thus need attention. In this paper, the phase stability model is developed from the basic Chen and Guo model (Chem Eng J, 1998, 71:145) to accurately predict the phase behavior of the clathrate hydrates of CH4 and CO2 in porous media of varying pore sizes from 6nm to 100nm which mimics the naturally occurring porous environment. We also propose a new equation for calculating the activity of water in porous media as a function of the pore size, the wetting angle, the surface tension, and the shape factor of the pores for varied temperature conditions. The model results are validated against experimental data available in open literature and found satisfactory. The proposed model uses very few input parameters (data intrinsic) and thereby is very beneficial in predicting the stability of the hydrates in virgin gas reservoirs wherein the characteristics of the gas reservoir are largely unknown. The developed model may further be applied to the hydrate systems of other natural gases in porous medium with suitable modifications. © 2014 Elsevier B.V.